1. Field of the Invention
The present invention relates to an improved adsorbent/process for the purification of atmospheric, industrial and other gases, and, more especially, to an adsorbent based on a molecular sieve of the zeolite type used in the processes of gas purification designated PSA or "Pressure Swing Adsorption" or TSA "Temperature Swing Adsorption". These methods entail passing the gas to be purified over an adsorbent under a high pressure and at a low temperature, then regenerating the adsorbent either by decompression (PSA) employing a very short cycle time on the order of a few minutes, or by heating (TSA), with cycle times generally on the order of a few hours.
This invention is particularly related to PSA and TSA processes wherein the proportion of adsorbed materials occupying the surface of the adsorbent is low. By "proportion of adsorbed materials occupying the surface of the adsorbent" is intended the ratio of the mass of the material adsorbed at a given pressure and temperature to the maximum amount of gas capable of being adsorbed on a predetermined volume of the adsorbent. In the PSA and TSA methods consistent herewith, the proportion of the surface of the adsorbent occupied is generally less than 15%.
2. Description of the Prior Art
The use of a zeolite of the 5A type for the purification of gases by the adsorption of CO.sub.2, CO, N.sub.2, NH.sub.3, hydrocarbons, etc., is known to this art. Compare, in particular, French Patent No. 2,232,511.
These compounds are often present as impurities in processes for the recovery of hydrogen, for example from cracked gaseous ammonia, or in natural gas reforming processes.
The aforesaid '511 patent describes a 5A type zeolite having an exchange coefficient between the Na.sup.+ cation and a divalent cation, advantageously calcium, of at least equal to 80%.
However, such zeolite, used without a binder, has a low degree of crystallization, resulting in a decrease in adsorption capacity. Furthermore, the preparation of a zeolite having enhanced capacity is relatively difficult and expensive.
It too is known to thus use type A zeolites mixed with a binder, for example a clay binder, such as montmorillinite, sepiolite or attapulgite. The adsorbent obtained in this manner is used for drying gases, desulfurization or the adsorption of CO.sub.2. However, the addition of a clay binder, while making it possible to reinforce the mechanical properties of the adsorbent, results in a significant reduction in the adsorbent properties thereof, in particular relative to certain compounds. This reduction, which is not dramatic for certain applications, such as drying, is in contrast a major disadvantage in the case of use of an adsorbent having a low exchange capacity (less surface occupied) such as in the PSA and TSA methods under consideration.